Satellite-Based Positioning System Reception Device Comprising a Function for Detecting False Lock-Ons

ABSTRACT

A satellite-based positioning system reception device comprises a function for detecting false lock-ons based on a linear combination of complex signals arising from correlation pathways between a signal received from a satellite and a prompt local code and a plurality of local codes shifted by determined delays with respect to the prompt local code. An advantage is that the function for detecting false lock-ons makes it possible to remedy the false lock-ons due to multi-path phenomena.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to foreign French patent applicationNo. FR 1101391, filed on May 5, 2011, the disclosure of which isincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a satellite-based positioning systemreception device comprising a function for detecting false lock-ons. Itapplies notably to satellite-based radionavigation systems, commonlydesignated according to the initials GNSS designating the Englishterminology “Global Navigation Satellite System”, and may be implementedin any navigation receiver.

BACKGROUND

A satellite-based positioning system or GNSS, comprises a plurality ofsignal emitters arranged on as many satellites forming a constellation.A minimum of four positioning satellites enable a mobile receiver thatcan process the signals received from them, to deliver position data forthe receiver, in terms of geographical coordinates (x,y,z) at adetermined instant t. The signals transmitted by the positioningsatellites occupy a wider bandwidth than that required by the throughputof data to be transmitted, with the aim of reducing the influence of theinterfering signals, and of reducing the spectral power density of thesignals transmitted in such a way that the latter are masked in thebackground noise. Thus, according to techniques which are in themselvesknown, the spectrum of the transmitted signals is spread, a carrier wavebeing modulated by a data signal overlaid on a pseudo-random noisespreading signal with high frequency, according to a periodic sequencespecific to each satellite.

According to these techniques, the satellite-based positioning,determined at the level of the receiver, consists firstly in detecting,in an acquisition step, the pseudo-random spreading codes modulating thesignals originating from the satellites. Each signal emitted by avisible satellite and received by the antenna of the receiver must thenbe demodulated by the receiver, so as to determine notably a propagationtime measurement and Doppler measurement.

Calculation means implemented in the receiver then make it possible tosynchronize, on the satellite signals received, locally generatedreplicas of these signals. Slaving is undertaken by a carrier loop,steering the phase of the local carrier, and by a code loop steering theposition, or phase, of the local code. This synchronization enables thereceiver to evaluate the propagation times of the signals originatingfrom the various satellites, and to deduce therefrom its position, byalso taking into consideration navigation data contained in the signals.

An acquisition phase typically makes it possible to initialize theoperation of the tracking loops, since neither the position, nor theDoppler frequency nor the code received are known a priori; but thetracking loops can operate only if the position of the code and theDoppler frequency are close to those of the useful signal of thesatellite considered. If one of the differences is too high, then a zerocorrelation no longer provides any information, and slaving may not beachieved. To carry out the acquisition phase, a search for a correlationpeak is performed between a local signal and the signal received, in atwo-dimensional space, by trying a plurality of assumptions regardingthe phase of the code and the value of the Doppler frequency, with asufficiently fine interval to allow detection of the correlation peak.As soon as a correlation peak has been found, the search for the codeand for the Doppler frequency may be refined by decreasing the searchinterval around the detected correlation peak. When the precisionobtained is considered to be sufficient, the loops are closed, andconverge by construction to the correlation maximum: a so-called“tracking” phase is then entered.

A major cause of positioning errors is related to the presence ofmultiple paths or “multi-paths” on the signals emitted by thesatellites. This phenomenon is related to the reflection of the wavesoff obstacles, for example buildings, the signal received then being acomposite signal consisting of direct signals and reflected signals.

With the aim of reducing multi-path errors, it is possible to resort,according to a technique in itself known, to a so-called “Double Delta”correlator. This solution is presented in the French patent publishedunder the reference FR 2739695, and is described in detail hereinafter.However, this solution presents a risk of false lock-ons, leading tosignificant or indeed unacceptable positioning errors, up to anorder-of-magnitude of a few tens of metres.

A risk of false lock-on is due to the existence of blind zones on theDouble Delta code discriminator used for the code loop. If the codeerror falls within a blind zone in which the code discriminator is zero,the code loop switches to open loop, thereby leading to a staticmeasurement error in the pseudo-range.

Such a risk also exists with other code discriminators exhibiting blindzones.

SUMMARY OF THE INVENTION

An aim of the present invention is to remedy at least the aforementioneddrawbacks, by proposing a satellite-based positioning device comprisinga function for detecting false lock-ons, making it possible notably toremedy the problems of false lock-ons due to a correlator of DoubleDelta type or any code discriminator exhibiting blind zones. It shouldbe noted in this respect that in the examples developed hereinafter,reference is made to a correlator of Double Delta type, it beingunderstood that the examples could also apply to other types of codediscriminators exhibiting blind zones.

For this purpose, the subject of the invention is a satellite-basedpositioning system reception device, comprising at least one receptionchannel configured to process a received digitized signal originatingfrom a microwave signal emitted by a determined satellite offering aninherent spreading code, each reception channel comprising at least:

-   -   means for generating a local carrier,    -   means for demodulating the received digitized signal by the        local carrier,    -   means for generating a punctual local code corresponding to the        said satellite spreading code, the generating means also        generating a plurality n of local codes shifted with respect to        the punctual local code by nonzero delays d_(i),    -   correlation means configured to determine signals representative        of coefficients of correlation between the signal received        demodulated by the local carrier and the local codes forming as        many correlation pathways,    -   the signals restored by the said correlation means being        processed by a correlation integrator providing complex signals        Z_(i) corresponding to respective correlation pathways,    -   the reception device being characterized in that each reception        channel furthermore comprises means for determining a function        for detecting false lock-ons which is determined on the basis of        the said complex signals according to the relation:

${{Detector} = \frac{{Re}\lbrack {Z_{I}{\cdot {\overset{\_}{Z}}_{P}}} \rbrack}{{Z_{P}}^{2}}},$

-   -   in which Z_(p) designates the complex signal arising from the        pathway for correlating the digitized signal received with the        punctual local code, and Z_(i) designates a linear combination        of the said complex signals Z_(i) associated with respective        weighting coefficients the said delays d_(i) and weighting        coefficients α_(i) satisfying the following relation:

$\{ {\quad\begin{matrix}{{\sum\limits_{i = 1}^{i = n}\alpha_{i}} = 0} \\{{\sum\limits_{i = 1}^{i = n}{d_{i} \cdot \alpha_{i}}} = 0.}\end{matrix}} $

In one embodiment of the invention, a correlation integrator can processthe signals restored by the said correlation means for a plurality ofdetermined correlation pathways, the complex signals Zi arising from thecorrelation integrator being processed by at least one codediscriminator providing a signal controlling a code loop, a plurality ofthe signals from among the complex signals Zi being utilized by thefunction for detecting false lock-ons.

In one embodiment of the invention, the reception device can comprisefive correlation pathways:

-   -   a punctual correlation pathway devised on the basis of the        prompt local code,    -   a late correlation pathway devised on the basis of a local code        shifted from the prompt local code by a delay d,    -   a very late correlation pathway devised on the basis of a local        code shifted from the prompt local code by a delay 2d,    -   an early correlation pathway devised on the basis of a local        code shifted from the prompt local code by a delay −d and    -   a very early correlation pathway devised on the basis of a local        code shifted from the prompt local code by a delay −2d,    -   the function for detecting false lock-ons being devised on the        basis of the complex signals and defined according to the        relation:

${Detector} = {\frac{{Re}\lbrack {( {{- Z_{TA}} + {2 \cdot Z_{P}} - Z_{TR}} ) \cdot {\overset{\_}{Z}}_{P}} \rbrack}{{Z_{P}}^{2}}.}$

In one embodiment of the invention, the reception device can comprisefive correlation pathways:

-   -   a prompt correlation pathway devised on the basis of the prompt        local code,    -   a late correlation pathway devised on the basis of a local code        shifted from the prompt local code by a delay d,    -   a very late correlation pathway devised on the basis of a local        code shifted from the prompt local code by a delay 2d,    -   an early correlation pathway devised on the basis of a local        code shifted from the prompt local code by a delay −d and    -   a very early correlation pathway devised on the basis of a local        code shifted from the prompt local code by a delay −2d,    -   the code discriminator devising a signal D_(code) formed by a        linear combination of the complex signals, respectively        designated Z_(P), Z_(R), Z_(TR), Z_(A), Z_(TA) arising from the        said correlation pathways according to the relation:

${D_{code} = \frac{{Re}\lbrack \{ {{2 \cdot ( {Z_{A} - Z_{R}} )} - {( {Z_{TA} - Z_{TR}} ) \cdot {\overset{\_}{Z}}_{P}}} \rbrack }{{Z_{P}}^{2}}},$

-   -   the function for detecting false lock-ons being devised on the        basis of the same complex signals and defined according to the        relation:

${Detector} = {\frac{{Re}\lbrack {( {{- Z_{TA}} + {2 \cdot Z_{P}} - Z_{TR}} ) \cdot {\overset{\_}{Z}}_{P}} \rbrack}{{Z_{P}}^{2}}.}$

In one embodiment of the invention, the reception device can furthermorecomprise comparison means configured to compare the value of the saidfunction for detecting false lock-ons with a threshold value.

In one embodiment of the invention, the reception device can comprisealert means, configured to be triggered when the said threshold value iscrossed by the said function for detecting false lock-ons.

In one embodiment of the invention, the reception device can furthermorecomprise switching means configured to activate a first codediscriminator when the said function for detecting false lock-ons isbelow the said threshold value, or a second code discriminator when thesaid function for detecting false lock-ons is above the said thresholdvalue.

In one embodiment of the invention, the reception device can furthermorecomprise switching means configured to activate a second codediscriminator when the said function for detecting false lock-onsexceeds a first threshold value, and to activate a first codediscriminator when the said function for detecting false lock-ons dropsback beneath a second threshold value below the said first thresholdvalue.

In one embodiment of the invention, the said first code discriminatormay be of narrow correlator type, and the said second code discriminatormay be of Double Delta correlator type.

In one embodiment of the invention, the said threshold value or valuesmay be predetermined values.

In one embodiment of the invention, the reception device can furthermorecomprise means for estimating the signal-to-noise ratio of the signalreceived, the said threshold value or values possibly being adjusted asa function of the signal-to-noise ratio estimated by the said means forestimating the signal-to-noise ratio.

In one embodiment of the invention, the reception device can furthermorecomprise noise filtering means which are applied to the function fordetecting false lock-ons.

In one embodiment of the invention, the reception device can furthermorecomprise means for estimating bias, formed by a second function fordetecting false lock-ons which is determined on the basis of complexsignals corresponding to additional correlation pathways formed on thebasis of local codes shifted mutually and shifted with respect to thepunctual local code by an advance or by a delay greater than 1 chip.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will becomeapparent on reading the description, given by way of example, offeredwith regard to the appended drawings which represent:

FIG. 1, a diagram illustrating by means of functional blocks, a digitalprocessing channel of a satellite-based positioning receiver;

FIGS. 2 a and 2 b, curves illustrating the characteristics of thecorrelation function, respectively with a conventional correlator, and acorrelator of Double Delta type;

FIGS. 3 a and 3 b, curves illustrating notably the characteristics of afunction for detecting false lock-ons, according to an exemplaryembodiment of the invention;

FIG. 4, a curve illustrating the characteristic of the correlationfunction with a correlator of narrow type.

DETAILED DESCRIPTION

The present invention proposes to dispose a specific correlator inparallel with the correlator of Double Delta type, with the aim ofdevising an indicator of false lock-on. This specific correlatorvanishes when the code error lies in the blind zone, and reaches itsnonzero maximum for a zero code error.

Thus when the indicator devised on the basis of this specific correlatorgoes for example below a certain threshold, a risk of false lock-on in ablind zone may be detected. In such a case it is possible to substitutetemporarily for the correlator of Double Delta type, for example anarrow correlator, enabling the code error to converge to zero, untilthe indicator goes back above the threshold.

This solution exhibits the advantage of not affecting the performance ofthe Double Delta in any way, as long as the indicator remains above thethreshold. The solution then allows fast convergence in the case offalse lock-on.

FIG. 1 presents a diagram illustrating by means of functional blocks, adigital processing channel of a satellite-based positioning receiver.

With reference to FIG. 1, a digital processing channel 1 receives asinput digitized signals originating from a satellite. A satellite-basedpositioning system reception device typically comprises a digitalprocessing channel for each satellite from which it receives a signal tobe processed. For example, a reception device can comprise twelvechannels, realized in a specific integrated digital electronic componentof ASIC type (according to the acronym corresponding to the Englishterminology “Application Specific Integrated Circuit”) or in aprogrammable digital component of FPGA type (according to the initialsdesignating the English terminology “Field Programmable Gate Array”). Adigital processing channel 1 comprises a hardware module 10 and asoftware module 12.

The hardware module 10 receives as input digitized signals originatingfrom a reception chain disposed upstream, and not represented in thefigure.

The hardware module 10 is configured to generate the local code and thelocal carrier, and to correlate them thereafter with the digitized inputsignal. It may be clocked at a relatively high frequency, for example ofthe order of 50 MHz. It comprises a local carrier phase digitalintegrator NCO_(P) 102 generating on the basis of a first appropriatecontrol signal the phase of the local carrier φ_(carrier). The phase ofthe local carrier φ_(carrier) allows the generation of a complex localcarrier on the basis of a carrier generator 104. The hardware module 10firstly allows demodulation of the carrier, by multiplication of thesignal digitized by the complex local carrier, by means of a multiplierM.

The hardware module 10 also comprises a code phase digital integratorNCO_(C) 101 controlled by a second appropriate control signal. The codephase digital integrator NCO_(C) 101 is configured to generate alocal-code phase φ_(code). The local-code phase φ_(code) controls alocal-code generator 103 implementing the spreading code specific to thesatellite to which the channel considered is dedicated. The hardwaremodule 10 secondly allows demodulation of the code, by multiplication ofthe carrier-demodulated signal arising from the aforementionedmultiplier M, by a local code managed by the local-codes generator 103.The local-code generator 103 can generate a plurality of local codes:the so-called “prompt” local code, as well as local codes advanced ordelayed with respect to the prompt local code. In the exampleillustrated by FIG. 1, the local-codes generator 103 generates a promptlocal code, a so-called “late” local code, corresponding to the promptlocal code delayed by a delay d for example equal to 0.5 chips, and aso-called “early” local code, corresponding to the prompt local codeadvanced by the delay d.

Thus, the previously mentioned demodulation of the code may be carriedout in parallel on various pathways corresponding to the various localcodes generated, by means of a plurality of correlation multipliers Mvdisposed in parallel. For example, in the case of a correlator of DoubleDelta type, the local-codes generator 103 can generate five local codes:a prompt local code, a late local code, an early local code, as well asa so-called “very late” local code corresponding to the prompt localcode delayed by a delay 2xd, and a so-called “very early” local codecorresponding to the prompt local code advanced by the delay 2xd.

The signals output by the correlation multipliers Mv can then beintegrated in a coherent manner by a correlation integrator 109,producing samples of digitized signals forming the correlation pathways.In the example illustrated by FIG. 1, three correlation pathways thusarise from the correlation integrator 109: a prompt pathway, a latepathway and an early pathway, which can be formalized by complexsignals, respectively: Z_(P)=I_(P)+jQ_(P), Z_(R)=I_(R)+jQ_(R) andZ_(A)=I_(A)+jQ_(A). These complex correlation signals can then beutilized by the software module 12. Thus, the correlation integrator 109may be clocked at a lower frequency, corresponding to the operatingfrequency of the software module 12, for example of the order of 50 Hz.In this way, the correlation integrator 109 carries out an integrationevery 20 ms; the latter may be an integrator with periodic reset to zerocommonly designated according the English terminology “Integrate andDump”.

The software module 12 makes it possible to generate the control signalsfor the local carrier phase digital integrator NCO_(P) 102 and for thecode phase digital integrator NCO_(C) 101, respectively via the slavingof a code loop DLL and of a carrier phase loop PLL.

The complex results arising from the correlation integrator 109 are usedby the software module 12 to slave the phase of the local carrier andthe phase of the local code, via the corresponding digital integrators102, 101, in such a way that the local carrier is in phase with thecarrier received, and that the local code of the prompt pathway is inphase with the code received, or stated otherwise that the code error iszero.

The code loop DLL comprises a code discriminator 121 receiving as inputthe various complex signals arising from the correlation pathways formedby the outputs of the correlation integrator 109, and providing asoutput a signal D_(code) representative of an estimation of the codeerror τ, that is to say of the difference between the phase of the codereceived φ_(code) _(—) _(recvd) and the phase of the local code φ_(code)_(—) _(local). The signal D_(code) representative of the estimation ofthe code error is provided to a code corrector 123, the lattergenerating speed-related code commands, at the operating frequency ofthe software module 12, that is to say for example every 20 ms. Thespeed-related code commands are applied to the input of a code-commandsamplifier 125 providing the second control signals in terms of number ofchips per sampling period to the code phase digital integrator NCO_(C)101.

A phase digital integrator or NCO produces a phase, in cycles for thecarrier, and in chips for the code. At each clock cycle or hech—1 hechcorresponding to a sampling period, i.e. the inverse of the samplingfrequency—the NCO increments the value of the phase by the value of thecontrol signal that it receives. The control signal is thus expressed incycles per hech or in chips per hech, calculated by the software module12 on the basis of a speed in metres per second. Thus, the gain of thecode-commands amplifier 125 can be written G_(code)=(F_(code)/c)/F_(e),F_(code) designating the frequency of the code, F_(e) the samplingfrequency and c the speed of light, and the gain of the carrier-commandsamplifier 126 can be written G_(carrier)=(F_(carrier)/c)/F_(e).

In a similar manner, the carrier phase loop PLL comprises a carrierdiscriminator 122 receiving as input the signal arising from the promptcorrelation pathway formed by the corresponding output of thecorrelation integrator 109, and providing as output a signal D_(carrier)representative of an estimation of the carrier phase error Δφ, that isto say of the difference between the carrier phase of the signalreceived φ_(carrier) _(—) _(recvd) and the local carrier phaseφ_(carrier) _(—) _(local). The signal D_(carrier) representative of thecarrier phase error is provided to a carrier corrector 124, the lattergenerating speed-related carrier commands, at the operating frequency ofthe software module 12. The speed-related carrier commands are appliedto the input of a carrier-commands amplifier 126 providing the firstcontrol signals in terms of cycles per sampling period to the localcarrier phase digital integrator NCO_(P) 102.

The present invention relates particularly to the code discriminator121. With a view to a better understanding of the present invention,notation designating various entities and physical variables isintroduced hereinafter, as well as mathematical relations applyingthereto.

For the rest of the account, the following notation is introduced:

C_(n)(φ_(code)) designates the periodic spreading code of a satellite n;

L_(code) designates the period of the code, in integer number of chips;

T_(code) designates the duration of a code slot—or chip—in seconds: thusthe product L_(code)×T_(chip) designates the period of the code, inseconds;

R(τ) designates the auto-correlation function of the code, thus:

R(τ)=1−|τ| if |τ|<1,

R(τ)=0 otherwise.

For a satellite n, R(τ) can also be written:

${R(\tau)} = {{R_{n,n}(\tau)} = {\frac{1}{L_{code}}{\int_{0}^{L_{code}}{{{C_{n}(u)} \cdot {C_{n}( {u + \tau} )}}\ {u}}}}}$

For two satellites n and p, the inter-correlation coefficient can bewritten:

${{R_{n,p}(\tau)} = {\frac{1}{L_{code}}{\int_{0}^{L_{code}}{{{C_{n}(u)} \cdot {C_{p}( {u + \tau} )}}\ {u}}}}},$

this coefficient having a zero or almost zero value when p is differentfrom n.

The signal received originating from a satellite at an instant t can beformulated according to the following relation:

S_(recvd)(t)=2 A. cos(φ_(carrier) _(—) _(recvd)(t))C_(n)(φ_(code) _(—)_(recvd)(t))+Σ(t), A designating the amplitude of the signal receivedand Σ(t) designating the signals originating from other satellites andfrom the noise.

The signal received can also be written according to the followingrelation:

S _(recvd)(t)=A. exp i(φ_(carrier recvd)(t))C _(n)(φ_(code recvd)(t))+A.exp i(−φ_(carrier recvd)(t))C _(n)(φ_(code recvd)(t))+Σ(t)

At an instant t, the local signals, respectively early, prompt and late,arising from the local-codes generator 103 can be formulated accordingto the following relations:

$\{ \begin{matrix}{{S_{{local}\; \_ \; {early}}(t)} = {\exp \; {{( {- {\phi_{{carrier}\; \_ \; {local}}(t)}} )} \cdot {C_{n}( {{\phi_{{code}\; \_ \; {local}}(t)} + d} )}}}} \\{{S_{{local}\; \_ \; {punctual}}(t)} = {\exp \; {{( {- {\phi_{{carrier}\; \_ \; {local}}(t)}} )} \cdot {C_{n}( {\phi_{{code}\; \_ \; {local}}(t)} )}}}} \\{{S_{{local}\; \_ \; {late}}(t)} = {\exp \; {{( {- {\phi_{{carrier}\; \_ \; {local}}(t)}} )} \cdot {C_{n}( {{\phi_{{code}\; \_ \; {local}}(t)} - d} )}}}}\end{matrix}\quad $

The sampled signals applied to the input of the correlation integrator109, corresponding respectively to the early, prompt and late localsignals can be written according to the following relations:

$\{ {\begin{matrix}{{Z_{A}(k)} = {\frac{1}{T}{\int_{kT}^{{({k + 1})}T}{{S_{recvd}(t)} \cdot {S_{{local}\; \_ \; {early}}(t)} \cdot \ {t}}}}} \\{{Z_{P}(k)} = {\frac{1}{T}{\int_{kT}^{{({k + 1})}T}{{S_{recvd}(t)} \cdot {S_{{local}\; \_ \; {punctual}}(t)} \cdot \ {t}}}}} \\{{Z_{R}(k)} = {\frac{1}{T}{\int_{kT}^{{({k + 1})}T}{{S_{recvd}(t)} \cdot {S_{{local}\; \_ \; {late}}(t)} \cdot \ {t}}}}}\end{matrix},{{where}\text{:}}} $

T designates the coherent integration time, in integer number of codeperiods.

The complex signals forming the correlation pathways respectively early,prompt and late can be written according to the following relations:

$\{ {\begin{matrix}{Z_{A} = {{{A \cdot \exp}\; {{({\Delta\phi})} \cdot {R( {\tau - d} )}}} + N}} \\{Z_{P} = {{{A \cdot \exp}\; {{({\Delta\phi})} \cdot {R(\tau)}}} + N}} \\{Z_{R} = {{{A \cdot \exp}\; {{({\Delta\phi})} \cdot {R( {\tau + d} )}}} + N}}\end{matrix},{\begin{matrix}{{the}\mspace{14mu} {term}\mspace{14mu} N\mspace{14mu} {corresponding}} \\{{to}\mspace{14mu} {the}\mspace{14mu} {{noise}.}}\end{matrix}\mspace{14mu} \begin{matrix}{{\Delta\phi} = {\phi_{{carrier}\; \_ \; {recvd}} - \phi_{{carrier}\; \_ \; {local}}}} & {{{phase}\mspace{14mu} {error}},} \\{\tau = {\phi_{{code}\; \_ \; {recvd}} - \phi_{{code}\; \_ \; {local}}}} & {{code}\mspace{14mu} {{error}.}}\end{matrix}}} $

In the case of a correlator of Double Delta or equivalent type, veryearly and very late pathways may be added, the corresponding complexsignals being formulated according to the following relations:

$\{ {\begin{matrix}{Z_{TA} = {{{A \cdot \exp}\; {{({\Delta\phi})} \cdot {R( {\tau - {2d}} )}}} + N}} \\{Z_{TR} = {{{A \cdot \exp}\; {{({\Delta\phi})} \cdot {R( {\tau + {2d}} )}}} + N}}\end{matrix}.} $

In the case of a “conventional” code discriminator, the characteristicrepresentative signal D_(code) may be defined according to the followingrelation:

$\begin{matrix}{{D_{code} = \frac{{Re}\lbrack {( {Z_{A} - Z_{R}} ) \cdot {\overset{\_}{Z}}_{P}} \rbrack}{{Z_{P}}^{2}}},} & (1)\end{matrix}$

where:

Re[ ] designates the real part and Z designates the conjugate value ofthe complex Z.

By neglecting the noise term N introduced previously, it is possible toreformulate the representative signal D_(code) according to thefollowing relation:

$\begin{matrix}{D_{code} = {\frac{{R( {\tau - d} )} - {R( {\tau - d} )}}{R(\tau)}.}} & (2)\end{matrix}$

Designating by R_(Delta)(τ) the numerator in relation (2) hereinabove,the latter can be written:

$\begin{matrix}{D_{code} = \frac{R_{Delta}(\tau)}{R(\tau)}} & (3)\end{matrix}$

The representative signal D_(code) characteristic of a correlator ofDouble Delta type may be defined according to the following relation:

$\begin{matrix}{{D_{code} = \frac{{Re}\lbrack {( {{- Z_{TA}} + {2 \cdot Z_{A}} - {2 \cdot Z_{R}} + Z_{TR}} ) \cdot {\overset{\_}{Z}}_{P}} \rbrack}{{Z_{P}}^{2}}},} & (4)\end{matrix}$

where:Relation (4) hereinabove can also be written in the following manner:

$\begin{matrix}{D_{code} = {\frac{{Re}\lbrack \{ {{2 \cdot ( {Z_{A} - Z_{R}} )} - {( {Z_{TA} - Z_{TR}} ) \cdot {\overset{\_}{Z}}_{P}}} \rbrack }{{Z_{P}}^{2}}.}} & (5)\end{matrix}$

By neglecting the noise term N introduced previously, it is possible toreformulate the representative signal D_(code) according to thefollowing relation:

$\begin{matrix}{D_{code} = {\frac{{2 \cdot ( {{R( {\tau - d} )} - {R( {\tau + d} )}} )} - ( {{R( {\tau - {2d}} )} - {R( {\tau + {2d}} )}} )}{R(\tau)}.}} & (6)\end{matrix}$

It should be noted that the value of d may be chosen typically less than0.25 chips in the case of a correlator of Double Delta type.

Designating by R_(Double) _(—) _(Delta) (τ) the numerator in relation(6) hereinabove, the latter can be written:

$\begin{matrix}{D_{code} = {\frac{R_{{Double}\; \_ \; {Delta}}(\tau)}{R(\tau)}.}} & (7)\end{matrix}$

The curves presented hereinafter are theoretical curves, all exhibitinga symmetry about the ordinate axis of the reference frames in which theyare represented.

FIGS. 2 a and 2 b present curves illustrating respectively thecharacteristics of the correlation function: R_(Delta)(τ) with aconventional correlator, and R_(Double) _(—) _(Delta) (τ) with acorrelator of Double Delta type.

With reference to FIG. 2 a, the latter presents a first curve 201representing the inter-correlation coefficient R as a function of thecode error τ. The first curve 201 corresponds to a theoretical case, andexhibits a triangular shape, being characterized by a first linear zoneincreasing from a zero minimum value to a maximum value when the codeerror τ varies between −1 chip and 0, followed by a second linear zonedecreasing down to a zero minimum value when the code error T variesfrom 0 to 1 chip.

A second curve 203 represents the inter-correlation coefficient R(τ+d)corresponding to a late pathway. The second curve 203 is a translationby d=0.5 chips of the first curve 201 leftwards along the abscissa axis.

A third curve 205 represents the inter-correlation coefficient R(τ−d)corresponding to an early pathway. The third curve 205 is a translationby d=0.5 chips of the first curve 201 rightwards along the abscissaaxis.

A curve R_(Delta)(τ) 200 represents the characteristic correlationfunction R_(Delta)(τ) of a conventional correlator, resulting from adifference between the first and second curves 203, 205 describedhereinabove. The correlation function R_(Delta)(τ) exhibits a zero valuefor the values of the code error τ of less than or equal to −1.5 chipsand greater than or equal to 1.5 chips. The curve R_(Delta)(τ) 200exhibits a first linear zone decreasing from 0 to a minimum value, whenthe code error τ varies between −1.5 chips and −d=−0.5 chips, and then asecond linear zone increasing from the minimum value to a maximum value,when the code error T varies between −d=−0.5 chips and +d=+0.5 chips,the value of the correlation function R_(Delta)(τ) being zero when thecode error τ is zero, and then a third linear zone decreasing from themaximum value to a zero value, when the code error τ varies betweend=+0.5 chips and 1.5 chips. A conventional correlator exhibits thedrawback of being very sensitive to multi-paths, inducing code errors ofpossibly greater than 100 metres.

Now with reference to FIG. 2 b applying to a correlator of Double Deltatype, this figure exhibits a first curve 211 representing the differencebetween the inter-correlation coefficient corresponding to the latepathway R(τ+d) and the inter-correlation coefficient corresponding tothe early pathway R(τ−d) as a function of the code error τ, thisdifference corresponding to the first term in the expression for thecorrelation coefficient R_(Double Delta)(τ) given in relation (6)hereinabove. The first curve 211 is characterized by a first zone, shortof a first value τ₁ equal to −1−d of the code error τ in the exampleillustrated by the figure, in which the function represented is zero.The first curve 211 then presents a first linear zone increasing up to amaximum value, when the code error τ varies from the said first value τ₁up to a second value τ₂ equal to −1+d, in the example illustrated by thefigure. The first curve 211 then exhibits a first positive plateau, whenthe code error τ varies between the said second value τ₂ and a thirdvalue τ₃ equal to −d in the example illustrated by the figure. The firstpositive plateau is followed by a second linear zone decreasing from themaximum value down to an opposite minimum value y, when the code error τvaries between the said third value τ₃ and a fourth value τ₄ equal toopposite +d, the function represented by the first curve 211 taking azero value when the code error τ is zero. The second decreasing linearzone is followed by a second plateau at the negative minimum value, whenthe code error τ varies from said fourth value τ₄ to a fifth value τ₅equal to 1−d, the opposite of the said second value τ₂. The secondplateau is followed by a third linear zone increasing from the negativeminimum value up to 0, when the code error τ varies between the saidfifth value τ₅ and a sixth value τ₆ equal to 1+d, the opposite of thesaid first value τ₁. In the zone situated beyond the said sixth value τ₆along the abscissa axis, the function represented by the first curve 211takes a zero value again.

A second curve 213 represents the difference between theinter-correlation coefficient corresponding to the very late pathwayR(τ+2d) and the inter-correlation coefficient corresponding to the veryearly pathway R(τ−2d) as a function of the code error τ, this differencecorresponding to the second term of the expression for the correlationcoefficient R_(Double) _(—) _(Delta)(τ) given in relation (6)hereinabove. The global shape of the second curve 213 is similar to thatof the first curve 211 described previously, in that the second curve213 also exhibits three linear zones: a first linear zone increasingfrom 0 to the positive maximum value when the code error T variesbetween a first value τ′₁ equal to −1−2.d and a second value τ′₂ equalto −1+2.d, a second linear zone decreasing from the positive maximumvalue to the opposite negative minimum value when the code error τvaries between a third value τ′₃ equal to −2.d and a fourth value τ′₄equal to opposite +2.d, and a third linear zone increasing from thenegative minimum value to zero when the code error τ varies between afifth value τ′₅ equal to +1−2.d and a sixth value τ′₆ equal to +1+2.d.The second curve 213 also exhibits a first plateau and a second plateau,the first plateau being at the positive maximum value, and extendingfrom the second value τ′₂ to the third value τ′₃, and the second plateaubeing at the negative minimum value, and extending from the fourth valueτ′₄ to the fifth value τ′₅, the function represented by the second curve213 taking a zero value short of the first value τ′₁ and beyond thesixth value τ′₆. The second curve 213 is distinguished from the firstcurve 211 in that the first value τ′₁ specific to the second curve 213is lower than the first value τ₁ specific to the first curve 211, thesecond value τ′₂ specific to the second curve 213 is greater than thesecond value τ₂ specific to the first curve 211, the third value τ′₃specific to the second curve 213 is lower than the third value τ₃specific to the first curve 211, the fourth value τ′₄ specific to thesecond curve 213 is greater than the fourth value τ₄ specific to thefirst curve 211, the fifth value τ′₅ specific to the second curve 213 islower than the fifth value τ₅ specific to the first curve 211, and thesixth value τ′₆ specific to the second curve 213 is greater than thesixth value τ₆ specific to the first curve 211.

A curve R_(Double) _(—) _(Delta)(τ) 210 represents the characteristiccorrelation function R_(Double) _(—) _(Delta)(τ) of a Double Deltacorrelator, resulting from a difference between the second and firstcurves 213, 211 described hereinabove.

The central part of the curve R_(Double) _(—) _(Delta)(τ) 210, on eitherside of the ordinate axis of the reference frame, exhibits globally thesame shape as the characteristic curve of the correlation functionR_(Delta)(τ) illustrated in FIG. 2 a by the curve R_(Delta)(τ) 200. Thezone corresponding to this central part is termed the “capture zone”. Aspecific feature and an advantage of a correlator of Double Delta typeis related to the fact that the correlation function R_(Double) _(—)_(Delta)(τ) takes a zero value in blind zones coinciding with theplateaus exhibited by the second curve 213 described previously. Anadvantage afforded by these blind zones is that signals arising frommulti-path phenomena exhibiting a delay encompassed within a blind zone,do not affect the code discriminator, and therefore have no effect onthe code error. Thus, a correlator of Double Delta type allows aninsensitivity to multi-paths whose delay is greater than 2.d. It shouldbe observed that beyond the blind zones, the discriminator enters anunstable zone, bringing the code error back within the blind zone if theabsolute value of the code error is lower than 1 chip, and pushesoutside otherwise. For code errors of greater than 1+2d chips inabsolute value, the correlation now provides only noise: the positioningreceivers can comprise a signal-to-noise ratio estimator advantageouslymaking it possible to detect such cases and to relaunch an acquisitionprocess.

According to a specific feature of the present invention, it is proposedto use the existing correlation pathways so as to devise a function fordetecting false lock-ons. The function for detecting false lock-ons maybe constructed, in the manner of a code discriminator, on the basis of acombination of the correlation pathways.

For example, it is possible, in a particular embodiment of theinvention, to devise the function for detecting false lock-ons by usinga linear combination of the pathways on which a correlator of DoubleDelta type is founded, that is to say on the basis of the threepathways, very early, prompt and very late. This particular embodimentexhibits the advantage of using the same correlation pathways as thoseused by the correlator of Double Delta or equivalent type, for tracking.

The function for detecting false lock-ons can according to thisparticular exemplary embodiment be regarded as a value formulatedaccording to the following relations:

$\begin{matrix}{{Detector} = {\frac{{Re}\lbrack {( {{- Z_{TA}} + {2 \cdot Z_{P}} - Z_{TR}} ) \cdot {\overset{\_}{Z}}_{P}} \rbrack}{{Z_{P}}^{2}}.}} & (8)\end{matrix}$

By neglecting the noise, relation (8) hereinabove may be reformulated inthe following manner:

$\begin{matrix}{{{Detector} = \frac{- ( {{R( {\tau - {2d}} )} - {{+ 2} \cdot {R(\tau)}} - {R( {\tau + {2d}} )}} )}{R(\tau)}},} & (9)\end{matrix}$

or equivalently:

$\begin{matrix}{{Detector} = {\frac{{2 \cdot {R(\tau)}} - ( {{R( {\tau - {2d}} )} + {R( {\tau + {2d}} )}} )}{R(\tau)}.}} & (10)\end{matrix}$

In relation (10) hereinabove, the terms expressing the correlationfunctions arising from local codes shifted with respect to the promptlocal code have been isolated from the terms expressing the correlationfunctions arising from the prompt local code.

Designating by R_(Detector)(τ) the numerator in relation (10)hereinabove, the latter can be written:

$\begin{matrix}{{Detector} = {\frac{R_{Detector}(\tau)}{R(\tau)}.}} & (11)\end{matrix}$

FIGS. 3 a and 3 b present curves illustrating notably thecharacteristics of a function for detecting false lock-ons, such asdefined by relations (8) to (11) hereinabove.

FIG. 3 a presents the curve R_(Double) _(—) _(Delta)(τ) 210 alreadydescribed previously with reference to FIG. 2 b. In the same referenceframe, is also presented a curve R_(Detector)(τ) 300 characteristic ofthe function for detecting false lock-ons, such as previously defined.

The curve R_(Detector)(τ) 300 is characterized by a triangular shapewithin the capture zone, and exhibits its maximum peak for a zero codeerror τ. Outside of the capture zone, the function R_(Detector)(τ)exhibits the specific feature of being zero, or less than 0. Notably thefunction R_(Detector)(τ) takes a zero value in the blind zones, prone tofalse lock-ons.

FIG. 3 b presents the characteristic curves of the various termscontained in the linear relation defining the function R_(Detector)(τ),that is to say of the various terms contained in the expression for thenumerator in relation (10) hereinabove.

A first curve 301 represents the characteristic of the term 2.R(τ). Thefirst curve 301 is similar to the first curve 201 described previouslywith reference to FIG. 2 a, with the exception that the slopes of theincreasing and decreasing linear zones are twice as large, and themaximum value of the peak is twice as high.

Two intermediate curves 302, 304 respectively represent thecharacteristic of the term R(τ+2d) and the characteristic of the termR(τ−2d). The shape of the intermediate curves 302, 304 is similar tothat of the two curves 203, 205 described previously with reference toFIG. 2 a, with the exception that the latter are translated by a delayequal to 2d.

A second curve 311 results from the sum of the two curves 302, 304 andrepresents the characteristic of the term R(τ−2d)+R(τ+2d). This is theterm comprising the correlation functions arising from local codesshifted with respect to the prompt local code, as expressed in relation(10) hereinabove, in which this term has been isolated.

The blind zones of the code discriminator correspond to the linear zonesof the auto-correlation function of the code. The present invention isbased on this principle, and proposes to devise a linear combination ofthe shifted correlation pathways taking a zero value in these linearzones, and a nonzero value for a zero code error, so as to define afunction for detecting false lock-ons. The general principle of thepresent invention is explained hereinafter.

According to the principle of the present invention, the function fordetecting false lock-ons must exhibit a zero value in the blind zones ofthe code discriminator used. In the general case where a plurality ofcorrelation pathways—each correlation pathway resulting from a localcode exhibiting a delay d_(i), positive (early) or negative (late), withrespect to the prompt local code—offer a plurality n of complex signalsZ_(i), the function for detecting false lock-ons may be regarded as avalue formulated according to the following relation:

$\begin{matrix}{{{Detector} = \frac{{Re}\lbrack {Z_{I} \cdot {\overset{\_}{Z}}_{P}} \rbrack}{{Z_{P}}^{2}}},} & (12)\end{matrix}$

where: Z_(i) designates a linear combination of the various complexsignals, i.e.:

$\begin{matrix}{{Z_{I} = {\sum\limits_{i = 1}^{i = n}\alpha_{i}}}{\cdot {Z_{i}.}}} & (13)\end{matrix}$

In relation (13) hereinabove, the terms α_(i) designate the weightingparameters, positive or negative integers.

In order that the function for detecting false lock-ons may be zero whenthe increasing or decreasing linear zones of the auto-correlationfunction or inter-correlation coefficient R(τ) correspond to the promptcorrelation pathway, it is necessary and sufficient that the followingrelations be satisfied:

$\begin{matrix}\{ {\begin{matrix}{{\sum\limits_{i = 1}^{i = n}\alpha_{i}} = 0} \\{{\sum\limits_{i = 1}^{i = n}{d_{i} \cdot \alpha_{i}}} = 0}\end{matrix}.}  & (14)\end{matrix}$

It is for example possible, in one embodiment of the invention, that thedetection function be based on the three traditional correlationpathways (prompt/early/late), and to choose the following weightingcoefficients and the following delays:

$\{ {\begin{matrix} {( {\alpha_{1};\alpha_{2};\alpha_{3}} ) = ( {{- 1};2;{- 1}} )} ) \\{( {d_{1};d_{2};d_{3}} ) = ( {{- d};0;{+ d}} )}\end{matrix}.} $

It is for example also possible for the detection function to rely onthe following choice of the following weighting coefficients and thedelays:

$\{ \begin{matrix} {( {\alpha_{1};\alpha_{2};\alpha_{3};\alpha_{4}} ) = ( {{- 1};1;1;{- 1}} )} ) \\{( {d_{1};d_{2};d_{3};d_{4}} ) = ( {{{- 2}d};{- d};{+ d};{{+ 2}d}} )}\end{matrix}\quad $

The function for detecting false lock-ons may be used with the aim ofdelivering an alert signal, when the function exceeds for example athreshold value.

Advantageously, the function for detecting false lock-ons may becompared with a threshold value, the crossing of the threshold valuebeing able for example to trigger the switching of the code loop from afirst code discriminator (for example a code discriminator of DoubleDelta type) to a second discriminator, for example of narrowdiscriminator type. A code discriminator of narrow correlator type isdescribed in detail, with reference to FIG. 4 hereinafter.

A code discriminator of narrow correlator type uses the early, promptand late correlation pathways. A narrow correlator may be defined in asimilar manner to a conventional correlator, notably by relations (1) to(3) presented previously. As opposed to a conventional correlator forwhich the delay d is equal to 0.5 chips, the latter is less in the caseof a narrow correlator, and is typically equal to 0.1 chips.

FIG. 4 presents a curve illustrating the characteristic of thecorrelation function with a code discriminator of narrow correlatortype.

In a manner similar to FIG. 2 a described previously, FIG. 4 presents afirst curve 401 representing the inter-correlation coefficient R(τ+d)corresponding to a late pathway. The first curve 401 is similar to thesecond curve 203 of FIG. 2 a.

A second curve 403 represents the inter-correlation coefficient R(τ+d)corresponding to a late pathway. The second curve 403 is similar to thethird curve 205 of FIG. 2 a.

A curve R_(Delta)(τ) 400 represents the characteristic of the functionR_(Delta)(τ) resulting from a difference between the second curve 403and the first curve 401. The function R_(Delta)(τ) takes a zero valuefor the values of the code error τ that are less than −1−d chips. Thefunction R_(Delta)(τ) then exhibits a first linear zone decreasing downto a negative minimum value, when the code error τ varies from −1−d to−1+d chips. The function R_(Delta)(τ) then exhibits a first negativeplateau at the said negative minimum value, when the code error τ variesfrom −1+d to −d chip. The first negative plateau is followed by a secondlinear zone increasing up to a positive maximum value, when the codeerror τ varies from −d to +d chips. The function R_(Delta)(τ) thenexhibits a second positive plateau at the said positive maximum value,when the code error τ varies from +d to 1−d chips. The second plateau isfollowed by a third linear zone decreasing down to 0, when the codeerror τ varies from 1−d to 1+d chips. For the values of the code error τthat are greater than 1+d chips, the function R_(Delta)(τ) takes a zerovalue.

The two plateaus are manifested as a limitation of the sensitivity ofthe code discriminator to multi-paths.

A mode of operation of a positioning receiver based on a function fordetecting false lock-ons according to one of the embodiments describedis now described.

Prior to the closure of the code loop DLL with a code discriminator ofDouble Delta type, on completion of the acquisition phase, the receivercan use a narrow correlator to ensure that the error converges in thecapture zone of the Double Delta discriminator. Comparison means, forexample implemented in the software module 12 described previously, canthen be configured to carry out a comparison between the value of thefunction for detecting false lock-ons with a threshold, and switchingmeans can for example allow a toggling over to the Double Deltadiscriminator when the value of the detection function passes above athreshold value, the narrow correlator continuing to be for example usedas long as the said value remains beneath the threshold value.

Advantageously, the switching means may be based on a hysteresisfunction, with the aim of limiting untimely switchings between thevarious discriminators. Thus, the switching means can for example allowa toggling over to the Double Delta discriminator when the value of thedetection function passes above a first threshold value, it subsequentlybeing possible for switchover to the narrow switch to be carried outonly if the detection function passes below a second threshold value,lower than the said first threshold value.

Advantageously again, the threshold value or the threshold values may bemodified in real time, for example if the receiver is equipped withmeans for estimating the signal-to-noise ratio: the threshold values canfor example be increased when the estimated signal-to-noise ratioincreases.

Advantageously, during tracking phases, it is possible to switch over tothe narrow correlator when the value of the function for detecting falselock-ons passes below a threshold value, for example during the periodrequired in order for the code error to come back to the capture zone ofthe Double Delta correlator. In a similar manner to the embodimentdescribed hereinabove, a function of hysteresis type can also beenvisaged, in such a way that the return to the correlator of DoubleDelta type may be triggered only when the detection function goes backabove a second threshold value.

It should be observed that the function for detecting false lock-ons maybe disturbed by noise, and the triggering of false alarms or ofundesirable switchings may be the consequence thereof. To remedy thisphenomenon, it is possible to increase the value of the threshold by amargin. However, in order to minimize the value of this margin withoutreducing the zone of tolerance on the code error, it is possible,advantageously, to apply appropriate filtering means to the function fordetecting false lock-ons, with the aim of reducing the influence of thenoise. The filtering means can for example consist of a first-orderfilter.

Furthermore, on account of the inter-correlation of the noise signalsbetween the various correlation pathways, for example prompt, very earlyand very late, the function for detecting the false lock-ons may exhibita bias with respect to the configurations devoid of noise. Such a biasmay impede the positioning of the value of the detection threshold.

Thus advantageously, additional means for estimating bias can allow anestimation of this bias, and the estimated value may be subtracted fromthe detection function so as to circumvent the bias phenomenon.

In order to estimate the bias in an optimal manner, that is to saynotably so that the bias estimation is not significantly affected by thepresence of the signal originating from the satellite and itsmulti-paths, the means for estimating bias can be based on additionalcorrelation pathways arising from local codes delayed or advanced withrespect to the prompt local code, by a delay d of greater than 1 chip.Thus, such additional correlation pathways will exhibit identicalinter-correlation properties, but will restore noise only. A functionfor detecting false lock-ons such as described previously, but based onthese additional correlation pathways, associated with appropriatefiltering, can form the means for estimating the bias. For example, twosimilar functions for detecting false lock-ons may be realized, a firstof them being based, in the manner of the embodiments describedhereinabove, on correlation pathways arising from prompt local codes andshifted with respect to the prompt local code, and a second being basedon correlation pathways arising from local codes shifted by a delay (orby an advance) determined by a delay of greater than 1 chip. Thefunction for detecting false lock-ons can then be equal to the result ofthe said first function from which the result of the said secondfunction is subtracted. The appropriate filtering can for example becarried out with a larger time constant on the second correlationfunction, with the aim of minimizing the noise before the aforementionedsubtraction operation.

It should be noted that such additional correlation pathways may alreadybe available in certain types of receivers, using multi-correlators forthe purpose of speeding up the acquisition phases.

1. A satellite-based positioning system reception device, comprising atleast one reception channel configured to process a received digitizedsignal originating from a microwave signal emitted by a determinedsatellite offering an inherent spreading code, each reception channelcomprising at least: means for generating a local carrier, means fordemodulating the digitized signal received by the local carrier, meansfor generating a prompt local code corresponding to the said satellitespreading code, the generating means also generating a plurality n oflocal codes shifted with respect to the prompt local code by nonzerodelays d_(i), correlation means configured to determine signalsrepresentative of coefficients of correlation between the signalreceived demodulated by the local carrier and the local codes forming asmany correlation pathways, the signals restored by the said correlationmeans being processed by a correlation integrator providing complexsignals Z_(i) corresponding to respective correlation pathways, eachreception channel furtherm comprising means for determining a functionfor detecting false lock-ons which is determined on the basis of thesaid complex signals according to the relation:${{Detector} = \frac{{Re}\lbrack {Z_{I} \cdot {\overset{\_}{Z}}_{P}} \rbrack}{{Z_{P}}^{2}}},$in which Z_(P) designates the complex signal arising from the pathwayfor correlating the digitized signal received with the prompt localcode, and Z_(i) designates a linear combination of the said complexsignals Z_(i) associated with respective weighting coefficients α_(i),the said delays d_(i) and weighting coefficients α_(i) satisfying thefollowing relation: $\{ {\begin{matrix}{{\sum\limits_{i = 1}^{i = n}\alpha_{i}} = 0} \\{{\sum\limits_{i = 1}^{i = n}{d_{i} \cdot \alpha_{i}}} = 0}\end{matrix}.} $
 2. The reception device according to claim 1,wherein a correlation integrator processes the signals restored by thesaid correlation means for a plurality of determined correlationpathways, the complex signals Z_(i) arising from the correlationintegrator being processed by at least one code discriminator providinga signal controlling a code loop, a plurality of the signals from amongthe complex signals Z_(i) being utilized by the function for detectingfalse lock-ons.
 3. The reception device according to claim 2, comprisingfive correlation pathways: a prompt correlation pathway devised on thebasis of the prompt local code, a late correlation pathway devised onthe basis of a local code shifted from the prompt local code by a delayd, a very late correlation pathway devised on the basis of a local codeshifted from the prompt local code by a delay 2d, an early correlationpathway devised on the basis of a local code shifted from the promptlocal code by a delay −d and a very early correlation pathway devised onthe basis of a local code shifted from the prompt local code by a delay−2d, the function for detecting false lock-ons being devised on thebasis of the complex signals and defined according to the relation:${Detector} = {\frac{{Re}\lbrack {( {{- Z_{TA}} + {2 \cdot Z_{P}} - Z_{TR}} ) \cdot {\overset{\_}{Z}}_{P}} \rbrack}{{Z_{P}}^{2}}.}$4. The reception device according to claim 2, comprising fivecorrelation pathways: a prompt correlation pathway devised on the basisof the prompt local code, a late correlation pathway devised on thebasis of a local code shifted from the prompt local code by a delay d, avery late correlation pathway devised on the basis of a local codeshifted from the prompt local code by a delay 2d, an early correlationpathway devised on the basis of a local code shifted from the promptlocal code by a delay −d and a very early correlation pathway devised onthe basis of a local code shifted from the prompt local code by a delay−2d, the code discriminator devising a signal D_(code) formed by alinear combination of the complex signals, respectively designatedZ_(P), Z_(R), Z_(TR), Z_(A), Z_(TA) arising from the said correlationpathways according to the relation:${D_{code} = \frac{{Re}\lbrack \{ {{2 \cdot ( {Z_{A} - Z_{R}} )} - {( {Z_{TA} - Z_{TR}} ) \cdot {\overset{\_}{Z}}_{P}}} \rbrack }{{Z_{P}}^{2}}},$the function for detecting false lock-ons being devised on the basis ofthe same complex signals and defined according to the relation:${Detector} = {\frac{{Re}\lbrack {( {{- Z_{TA}} + {2 \cdot Z_{P}} - Z_{TR}} ) \cdot {\overset{\_}{Z}}_{P}} \rbrack}{{Z_{P}}^{2}}.}$5. The reception device according to claim 1, further comprisingcomparison means configured to compare the value of the said functionfor detecting false lock-ons with a threshold value.
 6. The receptiondevice according to claim 5, further comprising alert means, configuredto be triggered when the said threshold value is crossed by the saidfunction for detecting false lock-ons.
 7. The reception device accordingto claim 5, wherein a correlation integrator processes the signalsrestored by the said correlation means for a plurality of determinedcorrelation pathways, the complex signals Z_(i) arising from thecorrelation integrator being processed by at least one codediscriminator providing a signal controlling a code loop, a plurality ofthe signals from among the complex signals Z_(i) being utilized by thefunction for detecting false lock-ons, and further comprising switchingmeans configured to activate a first code discriminator when the saidfunction for detecting false lock-ons is below the said threshold value,or a second code discriminator when the said function for detectingfalse lock-ons is above the said threshold value.
 8. The receptiondevice according to claim 5, wherein a correlation integrator processesthe signals restored by the said correlation means for a plurality ofdetermined correlation pathways, the complex signals Z_(i) arising fromthe correlation integrator being processed by at least one codediscriminator providing a signal controlling a code loop, a plurality ofthe signals from among the complex signals Z_(i) being utilized by thefunction for detecting false lock-ons, and further comprising switchingmeans configured to activate a second code discriminator when the saidfunction for detecting false lock-ons exceeds a first threshold value,and to activate a first code discriminator when the said function fordetecting false lock-ons drops back beneath a second threshold valuebelow the said first threshold value.
 9. The reception device accordingto claim 7, in which said first code discriminator is of narrowcorrelator type, and said second code discriminator is of Double Deltacorrelator type.
 10. The reception device according to claim 5, whereinsaid threshold value or values are predetermined.
 11. The receptiondevice according to claim 5, further comprising means for estimating thesignal-to-noise ratio of the signal received, wherein said thresholdvalue or values are adjusted as a function of the signal-to-noise ratioestimated by the said means for estimating the signal-to-noise ratio.12. The reception device according to claim 1, further comprising noisefiltering means which are applied to the function for detecting falselock-ons.
 13. The reception device according to claim 1, furthercomprising means for estimating bias, formed by a second function fordetecting false lock-ons which is determined on the basis of complexsignals corresponding to additional correlation pathways formed on thebasis of local codes shifted mutually and shifted with respect to theprompt local code by an advance or by a delay greater than 1 chip.